Self-piercing and clinching female fasteners, such as nuts, were developed by the predecessor in interest of the Applicant (Multifastener Corporation) about 50 years ago as disclosed, for example, in U.S. Pat. No. 2,707,322. Self-piercing and clinching nuts are attached to a metal panel in a die press, which may also be used to simultaneously form the panel, wherein generally an upper die platen includes an installation head and a lower die platen includes a die member or die button. In the case of a self-piercing nut, a projecting pilot portion pierces an opening in the panel and the panel metal adjacent the pierced panel opening is then clinched to the fastener with each stroke of the die press. In the case of a clinch nut, an aperture is pre-existing in the panel so that the projecting pilot portion is received in the aperture and the panel opening is then clinched to the fastener with each stroke of the die press.
More recently, self-piercing and clinching male and female fasteners were developed having tubular barrel portions as disclosed, for example, in U.S. Pat. No. 4,555,838 assigned to the Assignee of this Application. The self-piercing and clinching fasteners disclosed in this patent include a barrel portion that is tubular, and a radial flange portion that is integral with the barrel portion. These self-attaching male fasteners are installed in a die press, as described above, wherein the lower die platen includes a die button having a panel supporting surface, a central die post, and an annular die cavity surrounding the die post. The annular die cavity includes a semi-circular annular bottom surface, a radial lip portion at the upper extent of the semi-circular bottom surface, and a generally frustoconical surface extending from the radial lip portion to the end surface of the die button. It is very important in most applications of the self-piercing and clinching male fasteners that the fastener be able to withstand significant torque loads without twisting in the panel, which may destroy the fastener and panel assembly. One such method of increasing the torque characteristics of the male fastener has been drawing and slightly deforming the radial flange of the projection fastener into the panel with the upper die surface. However, this has proven to provide inadequate torque characteristics for many applications, including automotive applications.
Various attempts have been made to improve the torque resistance of self-piercing and riveting projection fasteners of the type disclosed in U.S. Pat. No. 4,555,838. Initially, anti-rotation protrusions or nubs were provided either on the barrel portion, or on the radial flange portion adjacent the barrel portion. As disclosed, for example, in U.S. Pat. No. 4,810,143, also assigned to the assignee of the present Application. Presently, self-attaching fastening elements of the type disclosed herein include a plurality of spaced pockets in the outer edge of the flange portion adjacent the barrel portion as disclosed in U.S. Pat. No. 5,020,950, also assigned to the assignee of the present Application. There remains, however, several problems associated with the use of pockets in the flange as disclosed in U.S. Pat. No. 5,020,950. First, the die surfaces which form the pockets wear, such that the pockets are not always fully formed in the flange portion resulting in insufficient torque resistance. Second, the self-piercing projection fastener and panel assembly is press sensitive. That is, if the panel metal is not fully deformed into the pockets, the torque resistance will be inadequate. Further, the pockets form stress risers in the panel which can become a source of failure of the fastener and panel assembly. Finally, the use of pockets in the flange portion may provide insufficient torque resistance, particularly where extreme torque resistance is required.
Attempts have been made to reduce the impact of the problems associated with the anti-rotation concepts detailed above. One such example is disclosed in pending U.S. patent application Ser. No. 10/004,918. This application discloses a radial flange portion having a radial surface defining spaced concave surfaces separated by an outer cylindrical surface. During installation of the fastening element to the panel, the barrel portion is deformed outwardly and upwardly defining a U-shaped channel that receives the panel. The radial flange deforms the panel downwardly into the U-shaped channel, thereby securing the fastening element to the panel. The outer cylindrical surfaces further deform the panel providing an interaction between the radial flange portion and the panel that produces anti-rotational qualities that increase the torque capabilities of the fastening element.
The arcuate design of the radial flange portion disclosed in the U.S. patent application referenced above has proven to increase the torque characteristics of the fastening elements. While the arcuate configuration is beneficial to deforming the panel due to the smooth transition of its arcuate shape, it is believed that the torque characteristics of the fastening element may be improved further by optimizing the design of the radial surface. Therefore, it would be desirable to provide a radial surface having a configuration capable of increasing the torque characteristics of the fastening element being mechanically attached to the panel.